Att. CPA's! How does the IRS look at and treat virtual/shared office space as my "registered" place of business

[Ana Nomys]

I want to move my business and business address out of my home office. I would like to rent shared office space where I can hang my business and professional licenses that also offers a shared assistant that will provide admin tasks like answering my phone, accepting my mail, preparing written correspondence and etc. My two questions are how much actual physical time must I spend their so it may be considered and treated as my actual "registered" place of business by the tax man? 

If I do this can i still use my home office (dedicated to business only) as a write off as well?